1. Field of the Invention
The present invention relates to an automatic gain control circuit which amplifies an analog signal while automatically controlling the gain of the signal, and more particularly, to an automatic gain control circuit which is used for mobile communication and which constantly maintains a signal-to-noise ratio of an output signal (a ratio of an output signal to noise).
2. Description of the Related Art
An automatic gain control circuit such as that described in Japanese Patent Application Laid-open No. Hei-4-037334 is known as an existing automatic gain control circuit. FIG. 8 shows the configuration of the existing automatic gain control circuit. An input IF signal is amplified by an amplifier 1 having a gain control function, and the thus-amplified signal is output to a demodulation circuit from an output terminal of the amplifier. At this time, part of the output from the amplifier 1 is input to an AGC detection circuit 6, and an AGC setting circuit 3 sets for the amplifier 1 a gain corresponding to the level of the output signal detected by the AGC detection circuit 6. If the level of the output signal is high, the automatic gain control circuit is controlled so as to reduce the gain of the amplifier 1, rendering the level of the output signal constant.
However, the foregoing configuration of the existing automatic gain control circuit is capable of constantly maintaining the level of the signal output but is impossible to constantly maintain a ratio of an output signal to noise (i.e., a signal-to-noise ratio). Since there is an increase in an AGC gain and the noise floor of the automatic gain control circuit, the ratio of an output signal to noise (i.e., a signal-to-noise ratio) of the output signal is deteriorated, as a result of which a bit error rate (BER) of the output signal is also deteriorated.
In mobile communication, as compared with a mean level, a receiving level changes as great as +10 dB or more to -30 dB or less. For this reason, in a case where the existing automatic gain control circuit is applied to mobile communication, a conversion level of the automatic gain control circuit is determined, in consideration of variations in the receiving level both in positive and negative ranges due to a fading phenomenon. If there is a drop in the receiving level in excess of the range of control of the automatic gain control circuit, the output signal is decreased, thereby deteriorating the ratio of the output signal to noise (i.e., the signal-to-noise ratio). This in turn deteriorates the bit error rate (BER) of the output signal at the time of a strong electric field fading phenomenon.